1. Field of the Invention
The present invention relates to paint circulation systems, and more particularly to methods of controlling the flow of paint through such systems.
2. Description of the Related Art
Paint systems are widely used to paint a range of manufactured articles, such as automobiles. It is common, in almost all paint circulation systems, to pump the liquid paint from a central supply station and then to distribute the paint along a supply and return channel. A number of “drop” lines, are provided between the supply and return channels. From the supply side at each drop line a small flow of paint is directed along the “drop line” through a pressure regulator/pressure reducing valve to a “Colour Change Valve” where a first fraction is diverted to a spray gun and the remaining second fraction is delivered to the return channel. This results in a paint circulation system with many parallel flow paths. The flow in each of these parallel drop lines must be above a minimum velocity. Otherwise, paint “settling” can occur which can cause two problems:                1) the settling can cause dirt which can block the drop line altogether, thereby preventing paint from reaching the paint spray gun; and/or        2) the settled or coagulated paint will make it through the paint spray gun and will appear on the finished product as dirt, requiring expensive remedial repair, in some cases.        
Of course, proper operation of these conventional paint circulation systems requires that they be configured so that the proper amount of paint is delivered to the each drop line. This starts with setting the minimum pressure on the return line to guarantee that enough pressure is available to deliver the target flow rate to the paint spray gun at the last drop (lowest pressure) on the system. This means that the last downstream connection between return line and a drop line is above that minimum pressure. This minimum “return line” pressure is set by a “back pressure” regulator at the central supply station.
Even though each drop line may be attached to a spray gun, the flow requirements between paint guns may change from one drop line to another. This might occur because a first spray gun may be used to spray a large area, therefore requiring a relatively high flow rate. On the other hand, a second spray gun may be used to spray a small area therefore requiring a relatively low flow rate. However, the first and second spray guns will usually have their own drop lines. This “drop line” pressure adjustment is conventionally made by a pressure regulator/pressure reducing valve which is located in the drop line and upstream of the CCV valve. The downstream pressure of the pressure regulator/pressure reducing valve must be set at a pressure higher than the pressure at the intersection of the drop joins and the return channel, by a value:                a) will overcome frictional pressure losses created by the paint at the correct flow rate;        b) will take into account “head” pressure changes as a result of any changes in elevation in the drop line between the downstream side of the pressure regulator/pressure reducing valve and intersection of the drop joins and the return channel.        
Bearing these parameters in mind, the pressure drop between the pressure regulator/pressure reducing valve and the return line is typically in the 2 psi range. The minimum return line pressure is typically in the 100 to 150 psi range.
This means that the downstream pressure on the pressure regulator/pressure reducing valve must be set in the 102 to 152 psi range. As an example, if the minimum return channel pressure were to increase by 1 psi, the flow rate would decrease by 50%. If the return channel pressure were to increase by just 2 psi, the flow rate would decrease by 100%, or in other words would be completely shut off, leading to failure. When the pressure at the return line node is equal to the set point of the pressure regulator/pressure reducing valve, the flow in the drop line will shut off.
Typically, the return channel pressures at each intersection with a corresponding drop line are difficult to predict and are even more difficult to measure during set up because they tend to vary with minor upstream and downstream changes. This means that the system needs to be “balanced” and this is usually attempted at start up with a number of iterative adjustments to provide substantially the same flow travelling through each of the drop lines.
Conventionally, the flow through each drop line is obtained by setting the pressure to be just above the pressure at the intersection of the drop line and the return line. The pressure at the drop line to return channel is typically not known. This is problematic, since small changes in any part of the system can create pressure changes at the intersection of the drop line and the return line which can quickly cause sudden reductions or sudden spikes in flow in one or more of the drop lines, almost at random, which make these conventional systems chronically unstable, difficult to balance initially and difficult to maintain in a balanced condition during operation.
Conventional paint distribution systems are also problematic when changing paints. Changes in paint viscosity can cause additional random pressure variations. Changes in viscosity will occur between batches of paint, the temperature of the paint, how accurately the operator in the central supply station mixes the paint with solvents, how much of those solvents evaporate between adjustments to the viscosity by the operator.
Minor adjustments to the pressure regulator/pressure reducing valve in one or more drop lines will usually in turn cause a domino effect and prompt other random changes in other drop lines as a result. The problem is further aggravated, when due to these imbalances, settling occurs causing partial or complete blockage of a drop line causing further changes in flow and pressure drops in the system.
When a regulator/pressure reducing valve paint circulation system requires a new paint colour to be put into the system (due to new models/colour mixes) the system must be purged and cleaned. This is typically done with various types of cleaning solutions with viscosities that are very different than a paint's viscosity. The system has not been balanced for these new viscosities. Accordingly, some drops will likely have no flow and some others too much flow. Consequently, it is very difficult to clean these systems effectively. In this case, the regulators in each drop line must be backed off to zero pressure, otherwise some drop lines will randomly have no flow or not enough flow. Given the lack of balance between drop lines in this condition, the shut-off valves on all drop lines except one must be closed. Then, one valve for one drop line must be opened at a time in order to ensure that each drop line is cleaned. Cleaning can involve generally four or five progressive cleaning steps and a typical circulation system can easily have 30 drop lines, making the cleaning process a large and expensive task.
When the new paint is introduced at a different viscosity it can take up to five days to balance these systems with two trained operators. To do this effectively a flow meter must be inserted sequentially in each drop line and the pressure regulator/pressure reducing valve in the drop line must be adjusted until the correct flow is obtained. Each time the next drop line is adjusted it affects the flow rate through the previously adjusted drops. Thus, many iterations are required until all drops are within an acceptable range. This is an expensive time consuming process.
Part of maintaining a high quality painted product is to ensure that no dirt is contained in the painted surface of the product. The regulator/pressure reducing valve has a large volume where the diaphragm is located in which the paint flow velocity drops below that which prevents settling. This causes settling and coagulation of paint to occur, and this “coagulated paint dirt” can then make its' way directly to the finished product.
In order to balance these systems by pressure, a pressure gauge assembly is installed upstream and downstream of the pressure regulator/pressure reducing valve. The assembly has a tee in the drop piping, then a 4″ to 6″ long pipe, then an isolation ball valve, then an isolation diaphragm, then the pressure gauge. There is no flow through these assemblies. Instead, the paint is “dead”. This again allows coagulation of paint creating dirt which, when it settles back into the main paint line, that can make its' way directly to the painted finished product.
The capital cost to provide the regulators/pressure reducing valves and pressure gauge assemblies and the labour to install them is a significant cost. The labour to remove and rebuild these assemblies as they wear out is significant.
In order to create the pressure drop, the regulator/pressure reducing valve has a needle valve which is actuated by a diaphragm. The orifice which is created by the needle valve and seat of the needle valve is very small. This creates a region of very high velocity fluid flow rate and correspondingly an area of very high shear rate. Also, there is very high energy dissipation in a tiny volume which creates a zone of very high energy density. When a paint containing metallic flake flows through this orifice the high energy dissipation and high shear rate crumple the flat “flake” into a ball. This ball does not reflect the light as flat flake will and causes a colour shift in the final painted product. The light is not reflected from the flake so the paint appears darker and dull as opposed to lighter with a sparkle.
Finished products that have dirt on them usually need to be repainted. Finished products that have been painted with damaged metallic flake may need to be totally repainted or scrapped. The repair of product and loss of product can be very expensive.
Paint is formed according to a precisely prepared paint formulation and usually includes a carrier and a number of additives to the carrier to provide the finished paint coating with a desired colour and with a desired finished effect such as a metallic or pearlescent effect, or a finish such as a high gloss. These additives are sophisticated and involve, in some cases, microscopic particles having a particular shape and particular multi-layered microlayers. It is not uncommon to encounter difficulties or inconsistencies in the finished paint coating which are believed to be caused, in part, by the damage to some of the additives through the orifice of the pressure regulator/pressure reducing valve.
Conventional paint systems are thus believed to require a relatively high capital cost. They are easily imbalanced resulting in more dirt and/or plugging, thereby requiring the difficult and time consuming job of flushing and rebalancing or recalibrating the system. Conventional paint systems thus have very high maintenance requirements and, given the above mentioned sensitivity to slight changes in the system, must be rebalanced if paint viscosity changes. This can involve, in some cases, a minimum of five eight hour shifts requiring a minimum of two manufacturing Associates.
Conventional paint systems by design create dirt which then makes its' way onto the final product causing expensive repairs. Conventional paint systems can destroy metallic paints if the paint is left too long in the system. Loss of the volume of paint contained in a paint circulation system is very expensive.
It is therefore an object of the present invention to address at least some of these problems.